Common cooking containers such as pots, frying pans, and baking tins are commonly fabricated from metal. People have become used to cooking in metal containers, both for the methods of cooking used and the taste and texture of the foods produced. More recently the use of microwave ovens has become popular, and because of the nature of microwaves, metal containers generally can't be used in such ovens.
In the last 20 years or so, as thermoplastic polymers (TPs) having better high temperature resistance have been developed, the use of these polymers for ovenware has been proposed, see for instance U.S. Pat. Nos. 4,626,557, 4,503,168, 4,585,823, 5,308,913, and 5,141,985, and European Patent Application 846,419, all of which are hereby included by reference. Similar items have been made from thermosetting polymers. These polymeric cooking containers can be used in thermal and/or microwave ovens and often can withstand the highest temperatures usually used in these ovens, for example about 290° C. (˜550° F.) or more. These containers have several advantages. They can be molded into practically any shape so that may be easily sealed and the contents can be refrigerated or frozen. Also, they are relatively difficult to break, and are relatively low in weight. However, when cooking food in these containers, particularly in a microwave oven, the cooking method (time and/or temperature for example) may have to be varied from the method used for a metal container, or the food will not normally have the same taste and/or texture. For example, bread or a casserole cooked in a plastic container in a microwave oven may not be browned on the outside surface. This is primarily due to the fact that in microwave ovens the heat is relatively uniformly transmitted into the depth of the items being cooked, as opposed to being conducted in from the surface. Also, in a microwave oven there are usually no hot surfaces to impart browning to the item being cooked.
In order to overcome the lack of browning in microwave ovens, as reported in World Patent Application 01/34720, ceramics containing susceptors have been incorporated into ovenware. Often, the susceptor-containing ceramic is in the form of a plate that is part of a ceramic piece of ovenware. This ovenware has the drawbacks of being heavy and brittle. Also, the materials for this ovenware are expensive and hard to form.
In Japanese Patent Application 63-141591 and World Patent Application 01/34702 it has been suggested that polymers resistant to high temperatures, especially liquid crystalline polymers (LCPs), be filled with materials that are susceptors. Materials that have been used include barium titanate and small amounts of carbon fibers. The resulting composition, when formed into ovenware and used in microwave ovens, is said to cause browning of the surfaces of the items being cooked that are in contact with the ovenware. These surfaces are heated because the susceptors in the polymer absorb microwave energy.
The use of various susceptor-containing compositions for use in microwave ovens is known. See for instance U.S. Pat. Nos. 5,021,293, 5,049,714, 4,518,651, 4,851,632 and 4,933,526. In the examples in these patents, the layer containing the susceptor is typically very thin.